GB471002A - Improvements in optical instruments - Google Patents

Abstract

471,002. Optical apparatus. WHITWELL, A., WHITWELL, L. E., and FINCHAM, E. F. May 29, 1936, No. 15153. [Classes 97 (i) and 98 (i)] An optical instrument is focused by dividing the field of view into two parts in such a manner that, when the instrument is not properly focused, one of the two parts appears blurred compared with the other and is also displaced relatively to the other, and when the instrument is properly focused both the relative blurring and the relative displacement vanish. The focusing means comprises a lens and a thick parallel plate of glass placed obliquely to the axis of the lens so that one of its non - refracting faces divides the field of view and the pupil of the observer's eye, thus enabling the observer to see part of the field of view through the lens alone and part through both lens and plate. Fig. 1 shows the elements of the invention, L being a lens having its focal plane at F and abc being a vertical linear object intersecting the axis of the lens and not lying in the focal plane. The thick parallel plate P is placed in the lower half of the field between the lens and the observer's eye E with its refracting faces inclined to the axis of the lens. Since the pencils emerging from the lens are not parallel the image seen by the observer consists of two lines one below the other and laterally displaced relatively to the other. If the object is moved into the focal plane, the rays emerging from the lens consist of parallel pencils and the image seen by the eye E is a continuous line. Fig. 5 shows the application of the invention to an ophthalmoscope, the plate P being positioned between the observer's eve E and the observed eye E<1> so as to split the field of view and the observer's pupil. If the observed eye is ametropic, e.g. hyperopic, the image of the fundus is formed at F.P. behind the retina and the observer sees the two halves of the field relatively displaced and one part blurred as above. described. The power of the lens L which has to be inserted in order to bring the two halves of the field into coincidence is then the error of the eye E<1>. Fig. 4 shows the application of the invention for focusing the objective of a camera, the lens L and plate P being fixed on the back of the camera in such a position that the focal plane of the lens coincides with the photographic sensitive surface when the latter is in position. The formation by the objective of an image in this plane is secured when the eye E sees the two parts of the image in coincidence as above described.